One common communication medium employs frequency division multiplexing.
In frequency division multiplexing, a signal from an input channel is usually shifted in frequency by a transmitter modulator. The shifted signal then occupies a different portion of the frequency band, the band having been allocated for transmitting signals over a communication path between the transmitter and a receiver. For example, U.S. Pat. No. 3,697,682; issued Oct. 10, 1972 and entitled "Video Signal Processing"; discloses a method and apparatus according to which fields of a video frame are frequency interleaved. Each frame comprises a succession of P fields. Each field represents a different group of scan lines and thus each field includes a plurality of scan lines. Also, each field is of equal field scan time duration T.sub.f. In scanning a frame, each field of the frame is delayed by a time equal to the scan time T.sub.f multiplied by a factor (P-i) where i represents the ordinal place of the field in the succession of fields which comprise the frame, i.e., i=1, 2, . . . , P. Thus, the first field of the frame is delayed a time T.sub.f multiplied by (P-1 ) while each succeeding field of the frame is delayed for a delay time equal to the delay time of the preceding field less the time of one field scan. Thereby, the respective fields coexist in time. Thereafter, each of the P fields is frequency shifted by a sinusoid having a frequency for shifting each field a different submultiple of the scan line frequency. As a result, the bandwidth of the resultant frequency multiplexed signal is essentially the same as that of the original video signal.
Another common communication medium employs time division multiplexing.
In time division multiplexing, the signal from an input channel modulates a train of pulses that are interleaved with the pulse trains of other channels. For example, the Bell System T1 carrier system interleaves a signal from each of 24 input channels in a respective one of 24 time slots of a resultant 1.544 megabits per second time multiplexed signal.
Thus, in frequency division multiplexing, the signal of each channel uses only a fraction of the bandwidth of the communication path but occupies the bandwidth all of the time. On the other hand, in time division multiplexing, the signal of each channel occupies the communication path only for a fraction of the time, i.e., during its time slot, but during that fraction of time the whole bandwidth is available to the signal.
Another form of multiplexing is called time compression multiplexing.
In time compression multiplexing, the signal from each input channel is stored for a short period of time. The signals from all channels are then read from the store, compressed in time, and transmitted sequentially, one after the other, over the communication path. See, for example, the publication by J. E. Flood et al, "Time-Compression-Multiplex Transmission", Proceedings of the Institution of Electrical Engineers, Vol. 111, No. 4, (April 1964), pp. 647-668.
Unfortunately, the time compression multiplexing art appears remiss in the processing of video signals.